1. Field of the Invention
The present invention relates to a current detection method for a voltage source inverter apparatus such as a PWM (Pulse Width Modulation) inverter which drives a three-phase alternating-current motor.
2. Description of the Background Art
FIG. 7 is a main circuit arrangement diagram of a conventional PWM inverter, showing an example wherein a current detection method for a voltage source inverter apparatus disclosed in, e.g., Japanese Laid-Open Patent Publication No. HEI4-54461, is employed. In this drawing, the numerals 1 to 3 indicate upper transistors, 4 to 6 denotes lower transistors, 11 to 13 designate upper diodes, 14 to 16 represent lower diodes, and the upper transistors 1 to 3 and the upper diodes 11 to 13 are referred to as an upper arm and the lower transistors 4 to 6 and the lower diodes 14 to 16 are referred to as a lower arm. The circuit has connection points, including a U-phase output terminal 21, a V-phase output terminal 22, a W-phase output terminal 23, a first input terminal 24 and a second input terminal 25. In addition, the circuit has a smoothing capacitor 30 and controls an alternating-current motor 31. Finally, 40 designates a current detector which detects the composite current of the lower transistors 4, 5, 6, and 106 represents a detection current detected by the current detector 40.
In the P inverter, one of the upper and lower transistors (for example, the transistor 1 and the transistor 4) is always ON and the other is always OFF. When it is assumed that the direction in which currents IU, IV, IW of the output terminals 21, 22, 23 in the U, V and W phases flow out of the inverter is positive, the sum total of these currents is always zero. When the lower transistor 4 is ON and the current IU is negative, IU flows in the lower transistor 4, but when the current IU is positive, IU flows in the lower diode 14. Accordingly, when it is assumed that the direction of flow from the emitter of the transistor 4 to the capacitor 30 is positive, the detection current 106 detected by the current detector 40 is represented by the following expression:
Detection current 106=-[(a current value when the transistor 4 is ON and IU is negative)+(a current value when the transistor 5 is ON and IV is negative) +(a current value when the transistor 6 is ON and IW is negative)].
A method of controlling an output voltage in the PWM inverter will be described in accordance with FIGS. 8 (a), 8(b) and 8(c) which are timing charts at a time when a voltage control ratio (hereinafter referred to as the "AMP"), as represented by the amplitude of a signal wave/amplitude of a triangular wave, is small. In this drawing, waveform of FIG. 8(a) shows relationships between the modulated amplitude of a triangular wave (hereinafter referred to as the "triangular wave") and the signal amplitude of sine waves (hereinafter referred to as the "signal wave"). Specifically, 100 indicates a triangular wave, 101 represents a signal wave, 102 denotes a signal wave which is identical to the signal wave 101 in amplitude and lags it by a phase angle of 120 degrees, 103 designates a signal wave which is identical to the signal wave 102 in amplitude and lags it by a phase difference of 120 degrees, and 105, 105a and 105b represent positive vertexes of the triangular wave. The waveform of FIG. 8(b) show the output current waveforms of the PWM inverter and the waveforms FIG. 8(c) show a detection current and the output current waveform of the PWM inverter having the highest absolute value, wherein 106 indicates a detection current of the current detector 40, and 107 indicated by a dotted line denotes a waveform having the highest absolute value among IU, IV and IW.
To control the output voltage in the PWM inverter, the triangular wave 100 and the signal wave 101 are compared and the upper transistor 1 is switched ON when the amplitude of the signal wave 101 is larger than that of the triangular wave 100. Reversely, when the amplitude of the signal wave 101 is smaller than that of the triangular wave 100, the lower transistor 4 is switched ON. The signal wave 102 and the triangular wave 100 are compared to control the transistors 2, 5 in the same manner. Also, the signal wave 103 is used to control the transistors 3, 6 in the same manner. By changing the cycles of the signal waves to change the inverter frequency and increase the amplitudes of the signal waves, the output voltage of the inverter can be increased. These signal waves 101, 102, 103 serve as the command values of the phase voltages of the U-phase output terminal 21, the V-phase output terminal 22 and the W-phase output terminal 23 and are equivalent to the fundamental wave components of the phase voltages of the PWM inverter.
When the amplitude of the triangular wave 100 is greater than those of the signal waves 101, 102, 103 as in FIG. 8, the three signal waves 101, 102, 103 and the triangular wave 100 always have points of intersection within one cycle of the triangular wave 100. Hence, the lower transistors 4, 5, 6 are all ON at the positive vertex of the triangular wave 100. For example, at the highest point 105a of the triangular wave 100, IU is negative, IV and IW are positive, and the sum total of the currents IU, IV, IW is zero. Accordingly, the current flowing in the transistor 4 can be detected by the current detector 40 at the highest point 105a of the triangular wave 100. As IU is positive and IV and IW are negative at the other highest point 105b of the triangular wave 100, the current detector 40 detects the current of the sum of IV and IW. This current of the sum of IV and IW is equal to the current IU and flows in the diode 14.
Also, when the AMP is smaller than 1, the output voltage is proportional to the AMP and the alternating-current output voltage effective value of the inverter is represented by approximately AMP*0.612*VDC (V) (here, VDC is a direct-current voltage applied to the terminals 24 and 25).
FIGS. 8(a), 8(b) and 8(c) are timing charts for a regenerative mode wherein energy is returned from the motor 31 in FIG. 7 to the inverter at the voltage control ratio AMP of 0.6 and at the output voltage/output current phase difference of 120 degrees. A direct-current power having polarity as indicated at the capacitor 30 is applied to the terminals 24, 25. At this time, the currents IU, IV, IW are assumed to have a form of sine waves. But even if the current waveforms are distorted, the same results are produced when the sum total of the currents in the three phases is zero and each phase difference between the corresponding currents is 120 degrees.
In any state, as described above, when the triangular wave 100 is larger than the signal waves 101, 102, 103, the maximum current flowing in the transistors or diodes can be detected at the highest point 105 of the triangular wave 100 in the conventional method. Since the sum total of IU, IV, IW is zero, the maximum current flowing in the transistors or diodes can be detected.
However, when the amplitudes of the signal waves 101, 102,103 approach the amplitude of the triangular wave 100, i.e., when the AMP is less than and close to 1, the period when the transistors 4, 5, 6 are ON at the same time may be extremely short. FIGS. 9(a), 9(b), 9(c), 9(d) and 9(e) are timing charts wherein the AMP is large (AMP=0.95). In this drawing, waveform of FIG. 9(a) shows relationships between the triangular wave and the signal waves, wherein 105c indicates the highest point of the triangular wave. Waveform of FIG. 9(b) shows a detection current and the output current waveform of the PWM inverter having the highest absolute value.
With respect to FIGS. 9, the detector 40 detects the current of the sum of IU and IV at point 105c, but since the period when the signal wave 101 exists below the triangular wave 100 is extremely short, the ON period of the lower transistor 4 is extremely short. Hence, the current of the current detector 40 also has a pulse of short width in a region where it matches the maximum value 107 of the phase current, and this cannot be detected practically.
In a conventional inverter, the AMP is often set to not less than 1 to cause voltage saturation in order to provide a large output voltage. In this case, though the output voltage is not proportional to the AMP, the rise in AMP increases the output voltage. FIGS. 10(a), 10(b), 10(c), 10(d) and 10(e) are timing charts at a voltage saturation time (AMP=1.5 and power-factor angle =120 degrees). In this drawing, waveform of FIG. 10(a) shows relationships between the triangular wave and the signal waves, wherein 105d denotes the highest point of the triangular wave 100. Waveform of FIG. 10(b) shows the output current waveforms of the PWM inverter and waveform of FIG. 10(c) shows a detection current and the output current waveform of the PWM inverter having the highest absolute value.
In FIGS. 10, only IU is negative and the signal wave 101 is always above the triangular wave 100 at the point 105d, the signal wave 101 and the triangular wave 100 have no point of intersection near the point 105d, and the transistor 4 is not switched ON at the point 105d, whereby the current waveform 106 of the current detector 40 is zeroed and the phase current of the inverter cannot be detected.
When the AMP is high in the conventional detection method as described above, detection will be inaccurate in a regenerative mode, i.e., when the phase difference between the voltage and the current is more than 90 degrees. Since a certain signal wave 101, 102 or 103 is positive and the highest (and detection time is short) in the regenerative mode, the phase current of the corresponding phase is negative (because the phase difference between the voltage and the current is more than 90 degrees), making detection difficult.
When the AMP is not less than 1, the triangular wave 100 and the signal wave 101,102 or 103 do not have a point of intersection at a position where the signal wave is positive and the highest. At this time, since the phase current value of the corresponding phase is negative in the regenerative mode, correct current detection cannot be performed. This also applies to a case where the sum of the currents of the upper transistors 1, 2, 3 is detected instead of the composite current of the lower transistors 4, 5, 6 which was detected in FIG. 7.
FIG. 11 is a main circuit arrangement diagram of the PWM inverter at a time when the composite current of the upper transistors 1, 2, 3 is detected, wherein 42 indicates a current detector and 10 designates a current waveform of the current detector 42. The detection current 110 detected by the current detector 42 is represented by the following expression: ##EQU1##
FIGS. 12(a), 12(b), 12(c), 12(d) and 12(e) are timing charts at a time when the voltage control ratio (AMP=0.95, power-factor angle =120 degrees) is large. Waveform of FIG. 12(a) shows relationships between the triangular wave and the signal waves, wherein 113 and 113a indicate negative vertexes of the triangular wave. Waveform of FIG. 12(b) shows the output current waveforms of the PWM inverter and waveform of FIG. 12(c) shows a detection current and the output current waveform of the PWM inverter having the highest absolute value. In this drawings, the current 110 matches the maximum value 107 of the phase current at the negative vertex 113 of the triangular wave, but the detection current pulse width decreases at a point such as the point 113a and detection is difficult as in the lower transistor current detector 40.
The present invention relates to a method for detecting the maximum value of the phase current of the inverter in either of the driving and regenerative modes.